To obtain hydrocarbons such as oil and gas, boreholes are drilled by rotating a drill bit attached to a lower end of a drilling assembly. Due to the very high cost of drilling such boreholes and the need to minimize the amount of time spent drilling and collecting borehole information, it is commercially advantageous to gain as much information as possible during the drilling process. Information about downhole conditions and materials may be acquired using wireline tools or logging-while-drilling (LWD) tool assemblies. Wireline tools can only be used after a portion of the borehole has been drilled and the drilling assembly has been removed. In contrast, LWD tool assemblies are integrated into the drilling string and may, therefore, be used while the borehole is being drilled. Downhole information acquired from LWD tools may be more immediately available, and can be used to monitor and adjust the drilling direction of the borehole, to detect the presence of geologic formations and hydrocarbons, or for any other purpose which would benefit advantageously from near-contemporaneous borehole information.
In the search for hydrocarbons, many formation properties are measured and analyzed. One tool that has been employed for both wireline and LWD applications is the acoustic logging tool, which is used to measure propagation velocities of acoustic waves in the formation. Measurements of compressional and shear wave velocities in a subsurface earth formation are reflective of formation densities, composition, fractures, and fluid saturation. In addition, the acoustic velocity measurement logs can be combined with seismic survey information to obtain an accurate structural model of nearby formations.
However, while acoustic LWD techniques have proven very successful in measuring compressional wave velocities, such techniques have had mixed success in measuring shear wave velocities. More specifically, while acoustic LWD techniques can successfully measure shear wave velocities in “fast” subsurface earth formations in which the shear wave velocity is greater than the borehole fluid velocity, the results are much less satisfactory in “slow” subsurface earth formations in which the shear wave velocity is slower than the borehole fluid velocity.
In performing acoustic LWD in subsurface earth formations, monopole, dipole and quadrupole-type source excitations have been used. As monopole shear waves generated by monopole-type acoustic LWD tools cannot propagate along the borehole wall in slow formations, monopole-type acoustic LWD tools are poorly suited for measuring shear wave velocity in slow formations. Because of the need to measure shear wave velocity in slow formations, particularly in the soft sediments of deep-water reservoirs, dipole-type acoustic LWD tools were developed.
Unlike (refracted) monopole shear waves, dipole shear waves (also known as borehole flexural waves) generated by dipole-type acoustic LWD tools are borehole guided. If dipole shear waves are generated in a sufficiently low frequency range (typically from about 1 kHz to about 3 kHz), they will travel at the shear wave velocity of the subsurface earth formation regardless of whether the subsurface earth formation is a fast subsurface earth formation or a slow subsurface earth formation. Accordingly, dipole-type acoustic LWD tools would appear to be well suited for measuring the shear wave velocity of slow subsurface earth formations.
Unfortunately, however, in a LWD environment, the tool body must be rigid and hence relatively massive. Because of the massive tool body, dipole-type acoustic LWD tool measurements are often adversely affected by the waves in the tool body itself. More specifically, measurements of the dipole shear wave traveling along the borehole tend to be severely contaminated by the dipole wave energy traveling in the tool body (the “tool mode”). As a result, dipole-type acoustic LWD tools may be less than ideal for measuring shear wave velocity of slow subsurface earth formations.
The final source excitation technique is of the quadrupole-type and is employed in quadrupole-type acoustic LWD tools. However, the measurements of the quadrupole or screw mode induced by the quadrupole-type acoustic LWD tool tend to be adversely affected by drilling noise. Specifically, at relatively low frequencies (less than about 3 kHz) where the quadrupole mode propagates with the formation shear wave velocity, the drilling noise overshadows the low excitation amplitudes of the quadrupole mode, making direct formation shear wave velocity measurements very difficult.
Accordingly, there exists a need for an acoustic LWD tool and an associated logging method to overcome the shortcomings of prior acoustic LWD tools.